During their self-assembly many bacteriophages and a number of eukaryotic viruses - including human herpesviruses and adenoviruses - package their double-stranded DNA genomes into pre-formed capsids by the action of a powerful ATP-dependent motor. Since it is believed that these viruses employ similar mechanisms to package DNA, the genome packaging process is a promising target for broad-spectrum anti- viral drug development. The packaging motor of bacteriophage ?29 is an ideal model system to investigate viral packaging due to a robust in-vitro packaging assay and extensive genetic, biochemical, structural, and single-molecule characterizations. Since this motor is comprised of a pentameric ring of ATPases that belong to the ASCE superfamily of ring NTPases, its study will also shed light on the operation of other members of this family that are responsible for a large number of cellular functions, such as ATP synthesis, chromosomal segregation, duplex unwinding, and protein unfolding. Our previous single-molecule studies allow us to build a comprehensive mechanochemical model for the ? 29 packaging motor and provide us with a unique opportunity to tackle fundamental mechanistic questions regarding motor operation with unprecendeted detail. In this application, we focus on the physical basis for the high degree of coordination and exquisite regulation observed in this motor. Specifically, we propose to: (1) dissect the mechanism of intersubunit coordination by monitoring wild-type motors under stressed conditions and mutant motors with deficient coordination phenotypes; (2) characterize the nature and strength of different types of contacts made between the DNA and the motor and the roles of these contacts in motor operation; (3) map the communication pathway between the DNA-filled capsid and the packaging ATPase and correlate the conformational dynamics of the motor complex to its packaging behavior. To carry out these studies, we will take advantage of state-of-the-art single-molecule instrumentation housed in our laboratory, including high-resolution dual-trap optical tweezers and a next- generation fluorescence-force hybrid microscope. Results of single-molecule biophysical measurements will be corroborated with genetic, biochemical, and structural studies through established collaborations. These interdisciplinary efforts will bring us closer toward a complete understanding of the viral packaging process and provide new opportunities for therapeutic intervention of viral infection.